1. Field of the Invention
The present invention relates to a surface acoustic wave filter apparatus for use as a bandpass filter and a branching filter including the surface acoustic wave filter apparatus, and more particularly, to a surface acoustic wave filter apparatus including a plurality of cascade-connected longitudinally coupled resonator SAW filters, and a branching filter including the surface acoustic wave filter apparatus.
2. Description of the Related Art
A surface acoustic wave filter is commonly used as a bandpass filter in mobile communication devices such as portable telephone devices, and is also used for various types of branching filters.
Japanese Unexamined Patent Application Publication No. 2003-249842 discloses a branching filter using a double-mode resonator type surface acoustic wave filter. FIG. 10 is a plan view schematically showing the structure of the branching filter disclosed in Japanese Unexamined Patent Application Publication No. 2003-249842.
The branching filter 501 includes a common terminal 502 for connection to an antenna, a transmitting terminal 503, and a receiving terminal 504. A first SAW (surface acoustic wave) filter apparatus 505 is connected between the common terminal 502 and the transmitting terminal 503, and a second SAW filter apparatus 506 is connected between the common terminal 502 and the receiving terminal 504.
In the SAW filter apparatus 505, a plurality of SAW resonators are connected so as to define a ladder circuit. On the other hand, the SAW filter apparatus 506 includes first and second SAW filters 507 and 508. The SAW filters 507 and 508 are 3-IDT resonator type SAW filters. Of the three IDTs (interdigital transducers) of each of the SAW filters 507 and 508, IDTs at the central location are commonly connected to the common terminal 502. One end of each of the IDTs at outer locations of the SAW filters 507 and 508 is commonly connected to the first receiving terminal 504. The other end of each of IDTs at outer locations of the SAW filters 507 and 508 is commonly connected to the second receiving terminal 509. The phase of a signal transmitted from the SAW filters 507 and 508 to the first receiving terminal 504 and the phase of a signal transmitted from the SAW filters 507 and 508 to the second receiving terminal 509 are different by 180° so that the SAW filter 506 has an unbalanced-to-balanced conversion function.
In the branching filter 501 disclosed in Japanese Unexamined Patent Application Publication No. 2003-249842, the SAW filter apparatus 506 in the receiving circuit is configured such that the two SAW filters 507 and 508 are connected together in parallel. This reduces insertion loss and improves the maximum allowable power.
Japanese Unexamined Patent Application Publication No. 2001-308672 discloses a SAW filter apparatus having a structure shown in FIG. 11. This SAW filter apparatus also has an unbalanced-to-balanced conversion function.
In this SAW filter apparatus 511, as shown in FIG. 11, SAW filters 515 to 517 are connected between an unbalanced signal terminal 512 and first and second balanced signal terminals 513 and 514. The SAW filter 515 includes IDTs 515a to 515c. One end of the IDT 515b at the central location is connected to the unbalanced signal terminal 512. One end of the IDT 515a is connected to the SAW filter 516, and one end of the IDT 515c is connected to the SAW filter 517. The SAW filters 516 and 517 are respectively connected to the first and second balanced signal terminals 513 and 514. In this SAW filter apparatus 511, to achieve an unbalanced-to-balanced conversion, the electrode structure is configured such that the phase of a signal flowing through the SAW filter 516 and the phase of a signal flowing through the SAW filter 517 are different by 180°.
In the SAW filter apparatus 501 disclosed in Japanese Unexamined Patent Application Publication No. 2003-249842 cited above, because a plurality of SAW filters 507 and 508 in the receiving SAW filter apparatus 506 are connected together in parallel, a piezoelectric substrate on which the SAW filters 507 and 508 are provided has a size large enough to accommodate these SAW filters 507 and 508 thereon, although it has the advantage that a reduction in the insertion loss in a passband and an improvement of maximum allowable power are achieved. That is, the total size of the SAW filter apparatus 506 increases, and thus, the total size of the branching filter 501 increases.
On the other hand, in the SAW filter apparatus 511 disclosed in Japanese Unexamined Patent Application Publication No. 2001-308672 cited above, although the unbalanced-to-balanced conversion is achieved, when the SAW filter 516 or the SAW filter 517 is used, for example, as a receiving filter of a branching filter in a portable telephone device, it is difficult to achieve high maximum allowable power.